This invention relates to batteries, and more particularly to a paste composition for lead acid batteries.
Lead acid batteries are the oldest and best-known energy devices in automobile applications. A common process to manufacture flat-pasted plate lead acid batteries is shown schematically in FIG. 1. Pure lead 10 is converted in step 20 to a 70-80% oxidized lead powder (lead oxide) in a Barton pot or a ball mill with a range of grain size distribution. For the positive paste, the mixing step 30 includes placing the dry lead oxide powder from step 20 in a positive mixing machine, such as a 3000 pound paste mixer, with water 40 and H2SO4 50 and mixing under constant stirring and at an elevated temperature. For the negative paste, mixing step 60 includes placing the dry lead oxide powder from step 20 in a negative mixing machine, such as a 3000 pound paste mixer, with water 40, H2SO4 50 and an expander 70 and mixing under constant stirring at ambient temperature. The pastes formed from mixing steps 30 and 60, depending on the ratio of starting materials, the rate of mixing and the temperature, contain mixtures of the initial powders, lead sulfate, and basic lead sulfates such as PbOPbSO4 (monobasic lead sulfate), 3PbOPbSO4xc2x7H2O (tribasic lead sulfate), and 4PbOPbSO4 (tetrabasic lead sulfate).
After a period of mixing, the pasting step 80 includes pressing the respective pastes on the expanded grids by a specially designed machine to prepare the positive and negative plates. To prevent sticking of the plates, the positive and negative plates are surface dried in an oven prior to stacking them on the skids, as indicated at steps 90,100 respectively. To improve the active material/grid contact and the mechanical strength of the active material, the skids with positive plates from step 90 are subjected to a steaming and curing process 110, which includes transporting the positive plates to a steam chamber for several hours and then to a curing room for about 3-4 days. During steaming and curing 110, further reaction of the ingredients occurs, resulting in a different ratio of the lead oxides, sulfate and basic lead sulfates. The resulting cured material is a precursor to lead dioxide, which forms the active material in the plates. The skids with negative plates from step 100 are also subjected to a curing process 115.
After curing is complete, the plates from steps 115 and 110 are transported to assembly 120 and a green battery is formed. The formation step 130 includes electrochemically oxidizing the precursor material for the positive electrode to lead dioxide and for the negative electrode to sponge lead, typically by adding sulfuric acid into the assembled cells. The finishing step, also 130, includes dumping the forming acid, refilling the batteries with the shipping acid, and sealing the batteries with a final cover.
The charging and discharging reactions for the positive and negative plates are as follows: 
During charging of the positive plate, the complex sulfates are converted into lead dioxide. This then becomes the positive active mass. When the cell is discharged, lead dioxide changes to lead sulfate. As the cell is recharged, lead sulfate changes back to lead dioxide, and this process keeps repeating. The negative plate construction is similar to the positive plate. The paste has similar ingredients and an additional material called an expander. The expander provides conditions during forming that result in the formation of high surface area lead xe2x80x9cspongyxe2x80x9d active mass at the negative collector surface. So, the negative cure paste starts out as sulfates etc., and during charging changes in to lead xe2x80x9cstructuresxe2x80x9d. When the cell is discharged, lead changes to lead sulfate. Thus, the negative active mass cycles between lead and lead sulfate.
When the batteries are constantly under-charged or deep discharged in application, a portion of the discharge product PbSO4 is never converted to the negative active material Pb on the negative plates. The crystal size of the unconverted PbSO4 increases during cycling, and larger void sizes may thereby form around the growing crystals. This results in shrinkage of the negative active mass, and thus degradation of the structure of the negative active mass. Eventually, the cell will lose its capacity to a point where it is no longer useful as a result of the shrinkage.
There is thus a need to develop a paste composition, and method to produce such paste, where shrinkage of the negative active mass is decreased during the cycling of the battery.
The present invention provides a paste composition for the negative plate, and a plate making process for a lead acid battery, which reduces shrinkage of the negative active mass during battery cycling. To this end, and in accordance with the present invention, a negative paste is produced by mixing an oxidized lead powder and sulfuric acid with an expander, a polymer and optionally carbon black to produce a paste comprising tribasic lead sulfate crystals, the expander and the polymer in which the reduction of Pb2+ to Pb during battery formation and cycling is facilitated by the polymer absorbed on the inactive lead sulfate crystals. After mixing the negative paste composition, the resulting paste is then pasted onto a grid where the paste is dried and cured to form a negative battery plate of the lead acid battery. The process and composition produces negative plates with reduced shrinkage in the negative active mass, and thus high performance and long cycling life for the battery.